The Plasma Membrane Membrane Transport

Figure 5.1 Figure 5.1 How do cell membrane proteins help regulate chemical traffic?

The Fluid Mosaic Model Phospholipids- the main “fabric” amphipathic= they have both hydrophilic AND a hydrophobic regions Proteins- embedded in the phospholipid membrane Also amphipathic Determine the function of the membrane Proteins are not distributed randomly or evenly, but rather according to function

How fluid is fluid? The membrane is held together my hydrophobic interactions-weaker than covalent bonds Constant lateral movement Proteins larger than lipids therefore move more slowly

Viscosity A measure of a fluid’s resistance to flow; how “thick” or “sticky” it is Due to molecular makeup and internal friction Honey is more viscous than water

What determines a membrane’s viscosity? Hydrocarbon tails on its phospholipids Saturated- more viscous Unsaturated- less viscous, more fluid Temperature Decrease in temp more viscous; may eventually solidify Increase in temp less viscous; too fluid, cannot support proteins and their function

What determines a membrane’s viscosity? Cholesterol- helps membranes resist changes in fluidity with changes in temperature High temps- restricts movement of phospholipids Low temps- prevents phospholipids from packing together Evolution Membrane composition evolves to meet specific environmental needs Cold

Unsaturated tails prevent packing. Saturated tails pack together. Figure 5.5 Fluid Viscous Unsaturated tails prevent packing. Saturated tails pack together. (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol reduces membrane fluidity at moderate temperatures, but at low temperatures hinders solidification. Figure 5.5 Factors that affect membrane fluidity Cholesterol 8

What determines a membrane’s viscosity? Evolution Membrane composition evolves to meet specific environmental needs Cold water fish Archea that live at 90°C (194°F) Some alter their composition seasonally

Membrane Proteins and Their Functions The proteins within the phospholipid bilayer determine the function of the membrane. Different cells  different membrane proteins Different organelles with a specific cell  different membrane proteins

Two Major Types of Proteins Integral Peripheral Can you see the difference?

EXTRACELLULAR SIDE OF MEMBRANE Figure 5.2 Fibers of extra- cellular matrix (ECM) Glyco- protein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Figure 5.2 Current model of an animal cell’s plasma membrane (cutaway view) Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

Integral Penetrates the membrane Transmembrane- through to both surfaces Partially embedded- only exposed on one surface The embedded portions have hydrophobic amino acids, often in an α helix Some have hydrophilic channels through them to allow for passage of substances through the membrane

N-terminus EXTRACELLULAR SIDE  helix CYTOPLASMIC SIDE C-terminus Figure 5.6 N-terminus EXTRACELLULAR SIDE Figure 5.6 The structure of a transmembrane protein  helix CYTOPLASMIC SIDE C-terminus 14

Peripheral Not embedded Bound to either surface Extracellular matrix (outside) Cytoskeletal elements (inside) Provide extra support for the membrane

EXTRACELLULAR SIDE OF MEMBRANE Figure 5.2 Fibers of extra- cellular matrix (ECM) Glyco- protein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Figure 5.2 Current model of an animal cell’s plasma membrane (cutaway view) Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

6 Major Functions of Plasma Membrane Proteins Transport Enzymatic activity Attachment to the cytoskeleton and ECM Cell-cell recognition Intercellular joining Signal transduction

(b) Enzymatic activity (c) Attachment to the cytoskeleton and extra- Figure 5.7 Enzymes ATP (a) Transport (b) Enzymatic activity (c) Attachment to the cytoskeleton and extra- cellular matrix (ECM) Signaling molecule Receptor Figure 5.7 Some functions of membrane proteins Glyco- protein (d) Cell-cell recognition (e) Intercellular joining (f) Signal transduction 18

Membrane Carbohydrates Cell-cell recognition Can be covalent bound to either lipids or proteins on the extracellular side of the membrane Glycoproteins Glycolipids Act as markers to distinguish cells Ex. ABO blood types

Membrane Synthesis Proteins and lipids- ER Carbohydrates added –Golgi

Selective permeability 2 aspects of “selectivity” The membrane takes up some small ions and molecules, but not others Substances that are allowed through, do so at different rates How does the membrane accomplish this selectivity?

Form Follows Function Hydrophilic head WATER WATER Hydrophobic tail Figure 5.3 Form Follows Function Hydrophilic head WATER WATER Figure 5.3 Phospholipid bilayer (cross section) Hydrophobic tail

Nonpolar substances= hydrophobic Cross easily Ex. Hydrocarbons, CO2 ,O2 Ions & polar substances= hydrophilic Hard to pass Ex. Glucose, H2O, Na+, Cl- Ions especially have a hard time as they tend to be surrounded by a “shell” of water molecules

Transport Proteins Channel proteins vs. carrier proteins Channel proteins create a channel through which hydrophilic substances may pass. Ex. Aquaporins Carrier proteins hold onto substances, change shape and redeposit them on the other side

Figure 5.14 EXTRACELLULAR FLUID [Na] high [K] low [Na] low 1 CYTOPLASM [K] high 2 ADP 6 3 Figure 5.14 The sodium-potassium pump: a specific case of active transport 5 4 25

Directionality of transport Controlled by Passive transport Diffusion Osmosis Facilitated diffusion Active transport Ion pumps, membrane potential Cotransport Bulk transport Exocytosis Endocytosis

Active transport Moves substances against their gradient; from an area of low concentration to one of high concentration Requires energy- supplied by ATP Allows cells to maintain a different environment inside vs. outside the cell

An example is the sodium- potassium pump

to the sodium-potassium pump. The affinity for Na Figure 5.14a EXTRACELLULAR FLUID [Na] high [K] low [Na] low CYTOPLASM [K] high ADP Cytoplasmic Na binds to the sodium-potassium pump. The affinity for Na is high when the protein has this shape. 1 Figure 5.14a The sodium-potassium pump: a specific case of active transport (part 1: Na+ binding) Na binding stimulates phosphorylation by ATP. 2 29

Phosphorylation leads to a change in protein Figure 5.14b Phosphorylation leads to a change in protein shape, reducing its affinity for Na, which is released outside. 3 Figure 5.14b The sodium-potassium pump: a specific case of active transport (part 2: K+ binding) The new shape has a high affinity for K, which binds on the extracellular side and triggers release of the phosphate group. 4 30

group restores the protein’s original shape, which has a Figure 5.14c Figure 5.14c The sodium-potassium pump: a specific case of active transport (part 3: K+ release) Loss of the phosphate group restores the protein’s original shape, which has a lower affinity for K. 5 K is released; affinity for Na is high again, and the cycle repeats. 6 31

Ion pumps maintain voltage across membranes Membrane potential= the voltage across a membrane Cytoplasmic side relatively negative Creates electrical potential energy that drives passive transport of cations into the cell and anions out Electrochemical gradient= chemical (concentration gradient) and electrical forces that drive diffusion across membranes

Main electrogenic pumps Animals- Sodium-potassium pump Plants- Proton pump

EXTRACELLULAR FLUID Proton pump CYTOPLASM Figure 5.16 Figure 5.16 A proton pump CYTOPLASM 34

Cotransport A process by which one protein transports 2 molecules or ions at a time. It uses the diffusion of solute to force the other against it’s gradient. It does not use ATP directly, but often is coupled with an ion pump that does use ATP

Sucrose-H cotransporter Figure 5.17 Proton pump Sucrose-H cotransporter Diffusion of H Figure 5.17 Cotransport: active transport driven by a concentration gradient Sucrose Sucrose 36

Bulk Transport Exocytosis Endocytosis Phagocytosis Pinocytosis Receptor-mediated endocytosis

Receptor-Mediated Endocytosis Figure 5.18 Receptor-Mediated Endocytosis Phagocytosis Pinocytosis EXTRACELLULAR FLUID Solutes Pseudopodium Plasma membrane Receptor Coat protein “Food” or other particle Coated pit Figure 5.18 Exploring endocytosis in animal cells Coated vesicle Food vacuole CYTOPLASM 38